1. Field of the Invention
The present invention relates to a time division transmission method between apparatus, such as data terminals, telephones and fascimiles, connected to a looped transmission line.
2. Description of the Prior Art
A channel multiplex system is well-known as the time division transmission method in a communication system in which a plurality of transmission apparatus 1,1 . . . are connected to a looped transmission line 2 and data terminals 3,3 . . . are connected to each transmission apparatus 1 as shown in FIG. 1.
The channel multiplex system makes frames 4,4 . . . of a format as shown in FIG. 2 circulate on the looped transmission line 2, so that the transmission apparatus 1 having data to be transmitted given from the data terminal 3, occupies slots of the same number, for example, those 6c,6c . . . , among a plurality of slots 6,6 . . . prepared in each frame 4, thereby sending out the divided data to the slot 6c at each frame 4. For a format of the frame 4 having five slots 6a to 6e as shown in FIG. 2, five transmission apparatus 1,1 . . . carry out the time division communication. Next, such system will be detailed.
FIG. 2 shows the format of one frame 4, in which reference numeral 5 designates a frame management region, 7 designates an access control region, and 6 designates a slot. The slot 6 has a first slot 6a through a fifth slot 6e and also each slot 6 comprises the access control region 7 at the head and a transmission unit 15 shown with magnification on the lower portion in FIG. 2, the transmission unit 15 is provided before and behind data 12 essentially to be transmitted by the transmission apparatus 1 with a transmission unit header 16 and a transmission unit trailer 17. The header 16 comprises regions 8, 9, 10 and 11 where a destination address DA, a sending address SA, the control information C and an effective data length LNG are to be set, respectively, the trailer 17 comprising regions 13 and 14 where an error correction code CRC and the response information RS are to be set. In addition, the transmission apparatus 1 is so set that a delay time until the data transmitted from the transmission apparatus 1 returns thereto is an integer multiple of a frame cycle period.
Next, explanation will be given on data transmission operation. In the channel multiplex system, any transmission apparatus 1 acquires the right of using the slot 6 according to the content of the access control region 7 provided at each slot 6, and thereafter, the transmission apparatus 1 can transmit data by use of the slot 6 of the same number at each frame 4 until the transmission apparatus 1 releases the right of using. In other words, the slot 6 of the same number at each frame 4 forms one channel for exchanging data between the transmission apparatus 1,1 . . . . FIG. 3 shows a case where five channels are provided.
In the channel multiplex system, the information of either "Free" or "Busy" is set in the access control region 7. When any transmission apparatus 1 acquires the right of using the slot 6, the access control region 7 thereof is set to "Busy" and the access control region 7 at the slot 6 whose right of using is not acquired by any transmission apparatus 1 becomes "Free".
The transmission apparatus 1, when requested of transmission, searches a slot 6 whose access control region 7 is set to "Free", and when the slot 6 set to "Free" is found, changes the slot set to "Free" to that set to "Busy" to thereby catch the slot 6 which is changed from being set to "Free" to "Busy", and the slots 6,6 . . . of the same number of the frame 4 whose slot is caught and the frames 4 subsequent to that are occupied, thus transmitting the date. Upon finishing the transmission, the access control region 7 of the slot 6 in use is returned to "Free" and then the slot 6 is released.
FIG. 3 shows an example of date transmission by the channel multiplex system. The transmission apparatus 1, when receiving the transmission data 18 to be transmitted from the date terminal 3, catches the slot 6 not in use, in other words, the slot 6 whose information of the access control region 7 is set to "Free". FIG. 3 is a view exemplary of the third slots 6c caught. Next, the transmission apparatus 1 divides the data 18 to be transmitted and each divided data is given the transmission unit header 16 and transmission unit trailer 17, thereby making the transmission unit 15. The transmission apparatus 1 sends out to the transmission line 2 the transmission units 15,15 . . . one by one with the timing of the third slots 6c at each frame 4. Upon finishing the transmission, an access control region 7 for each third slot 6c at the next frame 4 is set to "Free" and the third slots 6c,6c . . . are released.
Thus, in the conventional channel multiplex system, every transmission unit 15 is given the transmission unit header 16 comprising the destination address DA, sending address SA, and the transmission unit trailer 17 comprising the error correction code CRC and response information RS. Hence, an amount of annexed data required for transmission is larger than that of data to be essentially transmitted, thereby occurring the problem in that the transmission efficiency deteriorates.
Furthermore, in the channel multiplex system, the transmission apparatus 1 at the sending side divides the data 18 to be transmitted into the plurality of transmission units 15,15 . . . applied with the transmission unit headers 16,16 . . . and transmission unit trailers 17,17 . . . and transmits them. The transmission apparatus 1 at the receiving side must receive each transmission unit 15 to remove therefrom the transmission unit header 16 and transmission unit trailer 17, connect the successively received transmission units 15,15 . . . , and assemble them into the original data to be transmitted. Therefore, the transmission apparatus 1 is defective in complex construction due to the above processing. Especially, in a case where the transmission apparatus 1 receives the transmission units 15,15 . . . from the plurality of the transmission apparatus 1,1 . . . , the data 18 to be transmitted must be assembled respectively and simultaneously, thereby having the defect in that the processing becomes complex and the transmission efficiency deteriorates.